1. Field of the Invention
The present invention relates generally to the production of stream from geothermal brine and especially to such processes in which hot, pressurized, silica-rich geothermal brine is flashed to a reduced pressure to produce stream and in which the flashed brine is contacted with a seed material onto which silica is deposited from the brine.
2. Discussion of the Prior Art
Large subterranean aquifers of naturally produced (geothermal) steam or hot aqueous liquids, specifically water or brine, are found throughout the world. These aquifers, which often have vast amounts of energy potential, are most commonly found where the earth's near-surface thermal gradient is abnormally high, as evidenced by unusually great volcanic, fumarole or geyser activity. Thus, as an example, geothermal aquifers are not uncommon along the rim of the Pacific Ocean, long known for its volcanic activity.
Geothermal stream or water has, in some regions of the world, been used for centuries for therapeutic treatment of physical infirmities and diseases. In other regions, such geothermal fluids have long been used to heat dwellings and in industrial processes. Although efforts to further develop geothermal resources for these site-restrictive uses continue, considerable recent research and development has, additionally, been directed to exploitation of geothermal resources for production of electrical power which can be conducted, often over existing power grids, for long distances from the geothermal sources. In particular, recent steep increases in the cost of petroleum products used for conventional production of electric power, as well as actual or threatened petroleum fuel shortages or embargos, have intensified the interest in use of geothermal fluids as an alternative and generally self-renewing source of power plant "fuel".
General processes by which geothermal fluids can be used to generate electric power are known and have been known for some time. As an example, geothermal steam, after removal of particulate matter and polluting gases, such as hydrogen sulfide and ammonia, can be used in the manner of boiler-generated steam to operate steam turbine generators.
Naturally pressurized geothermal brine or water having a temperature of over about 400.degree. F. can be flashed to a reduced pressure to convert some of the brine or water to steam. The steam produced in this manner can then be used to drive steam turbine generators. The flashed geothermal liquid and the steam condensate obtained from power generation are typically reinjected into the ground to replenish the aquifer and prevent ground subsidence. Cooler geothermal brine or water can often be used to advantage in brine systems in which a low-boiling point, secondary liquid is vaporized by the hot geothermal liquid, the vapor produced being used to operate gas turbine generators. The cooled brine is typically reinjected into the ground.
As might be expected, use of geothermal steam is preferred over use of geothermal water or brine for generating electric power because the steam can be used more directly, easily and cheaply. Consequently, where readily and abundantly available, geothermal steam has been used for a number of years to generate commercially important amounts of electric power at favorable costs. For example, by the late 1970's, geothermal steam at The Geysers in Northern California was generating about two percent of all the electricity used in California.
While energy production facilities at important geothermal steam sources, such as at The Geysers, are still being expanded, when not already at capacity, the known number of important geothermal steam aquifers is small compared to that of geothermal brine or water. Current estimates are, in fact, that good geothermal brine or water sources are about five times more prevalent than are good sources of geothermal steam. The potential for generating electric power is, therefore, much greater for geothermal brine and water than it is for geothermal steam. As a result, considerable current geothermal research is understandably directed towards the development of economical geothermal brine and water electric power generating plants, much of this effort being expended towards the use of vast geothermal brine resources in the Imperial Valley of southern California.
Although, as above mentioned, general processes are known for using geothermal brine or water for production of electric power, serious problems, especially with the use of highly saline geothermal brine, have often been encountered in practice. These problems have frequently been so great as to prevent the production of electric power at competitive rates and, as a consequence, have greatly impeded the progress of flashed geothermal brine power plant development in many areas.
These severe problems are caused primarily by the typically complex composition of geothermal brines. At natural aquifer temperatures in excess of about 400.degree. F. and pressures in the typical range of 400 to 500 psig, the brine leaches large amounts of salts, minerals and elements from the aquifer formation, the brine presumably being in chemical equilibrium with the formation. Thus, although brine composition may vary from aquifer to aquifer, wellhead brine typically contains very high levels of dissolved silica, as well as substantial levels of dissolved heavy metals such as lead, copper, zinc, iron and cadmium. In addition, many other impurities, particulate matter and dissolved gases are present in most geothermal brines.
As natural brine pressure and temperature are substantially reduced in power plant steam production (flashing) stages, chemical equilibrium of the brine is disturbed and saturation levels of impurities in the brine are typically exceeded. This causes impurities and silica to precipitate from the brine, as a tough scale, onto surrounding equipment walls and in reinjection wells, often at a rate of several inches in thickness per month. Assuming, as is common, that the brine is supersaturated with silica at the wellhead, in high temperature portions of the brine handling system, for example, in the high pressure brine flashing vessels, heavy metal sulfide and silicate scaling typically predominates. In lower temperature portions of the system, for example, in atmospheric flashing vessels, amorphous silica and hydrated ferric oxide scaling has been found to predominate. Scale, so formed, typically comprises iron-rich silicates, and is usually very difficult, costly and time consuming to remove from equipment. Because of the fast growing scale rates, extensive facility down time for descaling operations may, unless scale reducing processes are used, be required. Associated injection wells may also require frequent and extensive rework and new injection wells may, from time to time, have to be drilled at great cost.
Therefore, considerable effort has been, and is being, directed towards developing effective processes for eliminating, or at least very substantially reducing, silica scaling in flashed geothermal brine handling systems. One such scale reduction process disclosed in U.S. Pat. No. 4,370,858 to Awerbuck, et al, involves the induced precipitation of scale-forming materials, notably silica, from the brine in the flashing stage by contacting the flashed brine with silica or silica-rich seed crystals. When the amount of silica which can remain in the brine is exceeded by the brine being flashed to a reduced pressure, silica leaving solution in the brine deposits onto the seed crystals. Not only do the vast number of micron-sized seed crystals introduced into the flashing stage provide a very much larger surface area than the exposed surfaces of the flashing vessels but also the silica from the brine tends to preferentially deposit onto the seed crystals for chemical reasons. Substantially all of the silica leaving the brine therefore precipitates onto the seed crystals instead of precipitating as scale onto vessel and equipment walls and in injection wells.
Preferably, the seed crystals are introduced into the high pressure flashing vessel, or crystallizer, wherein high pressure, two phase brine is separated. The silica removal or crystallization process, although commencing in the high pressure flash crystallizer, continues in successive, lower pressure flashing vessels in which additional two phase brine separation occurs. In a downstream reactor-clarifier, the silicious precipitate is separated from the brine as a slurry which may contain about 30 percent by weight of silica. According to known processes, a portion of this silicious slurry from the reactor-clarifier stage is recirculated back upstream into the high pressure flash crystallizer, wherein the silica material in the slurry acts as seed material.
After subsequent filtering to remove fine silicious particles not removed in the reactor-clarifier stage, the "clarifier" brine is commonly reinjected into the ground in an injection stage.
As above-mentioned, geothermal brines used for electric power generation are, at wellhead temperature and pressure, frequently saturated or even supersaturated with silica. As a consequence, substantial amounts of silica must be precipitated from the brine onto the seed material in the flash crystallization stage in order to prevent silica scaling in downstream brine handling equipment. Such removal of silica from the geothermal brine requires, particularly for high brine flow rates associated with production of reasonably large amounts of power, effective and rapid silica precipitation so that brine residence time in the flash crystallizer vessels, as determined by vessel capacity, can be maintained within acceptable and practical limits.
Such known silica seeding processes which utilize flash crystallizers to produce steam and control equipment scaling have, however, been found to have serious problems when implemented into a steam generating facility for providing geothermally derived steam to a power plant. Many of these problems relate to design and operation of flash crystallizers, in particular, high pressure flash crystallizer stages in which the geothermal brine is flashed to a substantially reduced pressure.
Flash crystallizers are typically constructed having an upright, cylindrically-shaped outer shell, in lower regions of which is installed a cylindrical draft tube. Provision is made for introducing brine into the crystallizer through the bottom of the draft tube. The upright draft tube is constructed sufficiently long (in the vertical direction) that upwelling of the brine and steam bubbles through the tube causes a natural, vertical circulation of the brine around and through the draft tube where the seed material is introduced (or is formed in situ). Such brine circulation, which is dependent upon-brine level in the crystallizer, is intended to provide prolonged contact of brine with the seed material, thereby affording time for silica to deposit from the brine onto the seed material. Ordinarily, flash crystallizers of the above-described type are intended to operate with a brine level at about the top of the draft tube. Differential pressure controlled valves, which may also be flashing valves, downstream of the flash crystallizer are provided for maintaining a given brine level in the crystallizer even though brine flow rates into the crystallizer may vary with time.
A space, usually about 6 to 10 feet high, is typically provided above the top of the draft tube in which the produced steam is demisted. In this regard, the associated power generating facility usually places limits on the amount of total dissolved solids (TDS) which may be present in the steam provided to it, the amount of TDS in the steam being generally indicative of the amount of brine carryover into the steam. When the allowed TDS level in the steam is exceeded, the power generating facility may be taken off line and the steam must then be discharged into the atmosphere, at a great waste of energy and at a great cost, until the steam TDS level is returned to within limits.
It has, however, been found in some operations that such draft tube-type, high pressure flash crystallizers have been unable to be operated at a brine level providing sufficient brine circulation to prevent excessive silica scaling of internal walls of the crystallizer without causing an excessive amount of brine carryover into the produced steam, as evidenced by excessively high steam TDS levels. Conversely, when the high pressure flash crystallizers are operated at brine levels which do not cause excessive brine carryover into the produced steam, brine circulation has ceased and the rate of silica scaling of inner vessel walls has been excessive, resulting in greatly increased costs of scale removal.
It has generally been supposed that the excessive brine carryover into the steam, at a brine level at the top of the draft tube, was an inherent characteristic of the brine steam separation process within high pressure flash crystallizers, there usually being no corresponding problems associated with low pressure flash crystallizers.
As a consequence, some new-design, high pressure flash crystallizers provide a substantially heightened space above the top of the draft tube to permit more height for steam demisting. Such designs, however, add greatly to the size and cost of the high pressure flash crystallizers and, upon installation, increase the cost of necessary piping.
Very importantly, the present inventors have determined that the problems associated with operation of preexisting high pressure flash crystallizers have unexpectedly been caused by severe foaming of the brine in the crystallizers, such foaming being greatly enhanced by the pressure within the crystallizer and by action of the steam bubbling through the brine after steam production. Such brine foaming had not before been seen in high pressure flash crystallizers because of their closed construction, and was unexpected because no significant amount of brine foaming had been seen in open portions of the steam producing facility.
Thus, the present inventors have discovered that when the brine level in the high pressure flash crystallizers was at the top of the draft tube, as intended, a thick blanket of foam extended upwardly from the top of the brine into the open region above the draft tube. The carryover into the produced steam, causing high steam TDS levels, was thus apparently a carryover of the foam. Further, it was discovered by the present inventors that because of the thick layer of foam produced in the flash crystallizer, the apparent brine level in the crystallizer, as determined by pressure measurements made at different crystallizer heights, was always greater than the actual height of the brine in the crystallizer. That is, the actual brine height was always lower than the indicated height, the indicated height being used, however, to control brine flow from the crystallizer. As a result, brine circulation in the crystallizer was always less than indicated or believed.
An object of the present invention is, therefore, to provide a process for treating geothermal brine in two phase brine separation vessels by reducing brine foaming.
Another object of the present invention is to provide a process for reducing the foaming of and the scaling by silica-rich geothermal brine in flash crystallizers, particularly in high pressure flash crystallizers.
Still another object of the present invention is to provide a process for reducing the foaming of and the scaling by a silica-rich geothermal brine in a high pressure flash crystallizer, the process including contacting the brine in the crystallizer with a defoaming agent and establishing and maintaining a brine level in the crystallizer which assures sufficient brine circulation to substantially reduce the silica scaling on inner walls of the crystallizer over that which would otherwise occur in the absence of brine circulation.
Other objects, advantages and features of the present invention will become apparent to those skilled in the art from the following description, when taken in conjunction with the accompanying drawings.